Chapter 5 Notes. 5.1 – Perpendiculars and Bisectors

Chapter 5 Notes

5.1 – Perpendiculars and Bisectors

Perpendicular Bisector Theorem, If a point lies on the perpendicular bisector of a segment, then the point is equidistant from the endpoints of the segment.

Converse of the Perpendicular Bisector Theorem, If a point is equidistant from the endpoints of a segment, then the point is on the perpendicular bisector of the segment.

A perpendicular bisector of a segment is a line or ray that is perpendicular to the segment at the midpoint.

A

BC

D

CDADthen

bisector on is D If

bisector on is Dthen

CDAD If

Distance from a point to a line is defined to be the length of the perpendicular segment from the point to the line (or plane)

Which one represents distance?

Angle Bisector Theorem, If a points lies on the bisector of an angle, then the point is equidistant from the sides of the angle.

Converse of the Angle Bisector Theorem, If a point is equidistant from the sides of an angle, then the point lies on the angle bisector.

A

B

C

D

DCBCthen

bisector on is C If

bisector on is Cthen

DCBC If

Constructing a perpendicular to a line through a given point on the line.1) From the given point, pick any arc and mark the circle left and right.

2) Those two marks are your endpoints, and construct a perpendicular bisector just like the previously.

Justification. Line is perpendicular by construction, 3 is on the bisector because it is equidistant to both endpoints (because radii are equal), so the line is going through the point.

D

K

C

U

S

Which segment is the perpendicular bisector, how do you know?

Find DK.

Find US.

Find SK.

Find CK

8

512

What could DK be so that the segment would NOT be a perpendicular bisector, how would you know?

O M

T

E

G

68

R

Y

R lies on what? How do you know?

OM is the angle bisector of EOT

Find MT.

OM

T

E

G

R

b8

30oxo

yo

a

6

5.2 – Bisectors of a Triangle5.3 – Medians and Altitudes of a

Triangle

Where multiple lines meet is called the point of concurrency. The lines that go through that point are called concurrent lines.

Thrm: The angle bisectors of triangle intersect in a point that is equidistant from the three sides of a triangle.

The point of concurrency of angle bisectors is called an INCENTER

Justification, points on angle bisector are equidistant to the sides, then transitive.

Thrm: Perpendicular bisectors of the sides of a triangle intersect in a point that is equidistant to all the vertices.

The point of concurrency of perpendicular bisectors is called a CIRCUMCENTER

Justification, points on perpendicular bisector are equidistant to the endpoints, then transitive.

So to help keep track of things, it’s like they go with the other, angle bisectors equidistant to sides. Perpendicular bisectors equidistant to vertices.

Inside or outside, where do the points of concurrency meet? Make a sketch and see

CIRCUMCENTERS INCENTERS

Acute – Inside

Right – On side

Obtuse – Outside

All inside

MB = x2 – 8

MA = -7x

M A

B

Red lines are angle bisectors.

143x - 4

Blue lines are perpendicular bisectors

5

Median – A line from the midpoint to the vertex

Where they all meet is the CENTROID

The distance from the Centroid to the vertex is 2\3 the median.

The distance from the Centroid to the midpoint is 1\3 the median.

DU =

CK =

DM =

SM =

CU =KS =

US =DC =

DS =

CS =UG =

GK =

DK =

DG =

GS =

CM =

CG =GM =

5

18

9

24

6

6

1:2

Think D

U

C

GM

K S

Thrm: Altitudes all meet at point.

Nothing special about it.

The point of concurrency of altitudes is called an ORTHOCENTER

Inside or outside, where do the points of concurrency meet? Make a sketch and see

Orthocenters Centroids

Acute – Inside

Right – On vertex

Obtuse – Outside

All inside

5.5 – Inequalities in One Triangle

Terminology and Concepts

Terminology The side opposite the angle is the side that is across from and doesn’t touch the angle.

Concept The sides and angles opposite from each other often relate to each other. Angles will use an uppercase letter, and the side opposite will use a lower case letter or segment name.

A

BCa

c b

Theorem 5.10 If one side of a triangle is longer than a second side, then the angle opposite the first side is larger than the angle opposite the second side. R

T STmSm :Prove

RSRT RST; :Given

Theorem 5.11 If one angle of a triangle is larger than the 2nd angle, then the side opposite the first angle is longer than the side opposite the 2nd angle.

RSRT:Prove

TmSm RST; :Given

Basically, big angle goes with big side, small angle goes with small side.

Exterior angle inequality theorem: The measure of an exterior angle of a triangle is greater than the measure of either of the two nonadjacent interior angles.

NONADJACENT means not attached to.

R

T S

Triangle Inequality Theorem: The sum of the lengths of any two sides of a triangle is greater than the length of the third side.

AB + BC > AC

AC + BC > AB

AB + AC > BC

A

B C

Pick the greater angle, 1 or 2?

2

9

8

1111.1

Name the sides, shortest to longest.

R

T S

50o____ < ____ < ____

Is it possible for a triangle to have these side lengths?

5, 6, 710, 10, 101, 1, 21.1, 1.2, 1.34.9, 5, 10

Given two side lengths, find the possible lengths for the 3rd side ‘x’

5, 62, 101, 9

5.6 – Indirect Proof and Inequalities in Two Triangles

B

A

C

E

D

F

Hinge Theorem If two sides one triangle are congruent to two sides of another triangle, but the included angle of the first triangle is larger than the included angle of the second, then the third side of the first triangle is longer than the third side of the second triangle.

Fancy talk for two sides same, one angle bigger than other, then side is bigger

DFAC :Then

EmBm

,EFBC,EDBA :Given

B

A

C

E

D

F

Converse of Hinge Theorem If two sides one triangle are congruent to two sides of another triangle, but the 3rd side of the first triangle is longer than the 3rd side of the second, then the included angle of the first triangle is larger than the included angle of the second.

Fancy talk for two sides same, one sidee bigger than other, then angle is bigger

EmBm :Then

DFAC

,EFBC,EDBA :Given

Lots of examples of both types, along with algebra styles

U

DC

K

S

30o45o

35o

List the angles and sides in order

____ < ____ < ____

____ < ____ < ____

student

U

DC

K

S

1

70o

2

70o

13

14

____ < ____

____ < ____

Indirect Proof

How to write an indirect proof1. Assume temporarily that the conclusion is not

true.

2. Reason logically until you reach a contradiction of the known fact.

3. Point out the temporary assumption is false, so the conclusion must be true.

Practice Write the untrue conclusion

odd isn :Prove ABAB :Prove

180 is trianglea of angles

interior of sum :Prove genius a is Kim Mr. :Prove

1

3 b||a :Prove

3m1m:Given a

b

pairlinear anot are 2 and 1 :Prove

602m,051m:Given

13 b||a :Prove

supp.not 31,:Given a

b